Various techniques are available for saccharification of cellulose, among which an enzymatic saccharification method, which uses mild reaction conditions and achieves high sugar yield, has become the mainstream of development.
Cellulase, which is a cellulose degrading enzyme, is roughly classified into cellobiohydrolase, which acts on the crystalline regions of cellulose, and endoglucanase, which reduces the molecular weight by acting on within the cellulose molecular chain. These cellulases are known to be inhibited by cellobiose, which is one of the products of cellulose degradation. Meanwhile, β-glucosidase is an enzyme that acts on a water-soluble oligosaccharide or cellobiose and catalyzes a hydrolysis reaction of the β-glycosidic bond. Particularly, β-glucosidase is an enzyme necessary for the acquisition of plenty of glucose, which is useful as a fermentation raw material. Also, it is known that the reactions mediated by cellobiohydrolase or endoglucanase are inhibited by the accumulation of cellobiose, which is produced by cellulose degradation. That is, β-glucosidase has an effect of greatly improving the cellulose degradation efficiency, owing to its capability of drastically reducing the accumulation of cellobiose produced by cellulose degradation.
Cellulose is contained abundantly in herbaceous plants and woody plants, which are collectively called cellulosic biomass. Cellulosic biomass contains, in addition to cellulose, hemicellulose such as xylan and arabinan, and lignin. Particularly, being an aromatic polymer compound, lignin contained in cellulosic biomass is known to act in an inhibitory manner in the enzymatic saccharification by cellulase derived from filamentous fungi. Although the mechanism of inhibition of cellulase derived from filamentous fungi by lignin has not been entirely elucidated yet, the reduced degradation efficiency caused by adsorption of cellulase to lignin is proposed as one of the causes (P. Hetti et al., Journal of Biotechnology, 107, 65 to 72 (2004)).
A heat-resistant enzyme is highly stable and retains its activity for a long time even under high temperature conditions; therefore, the application of a heat-resistant enzyme as an industrial enzyme is under study. A large number of heat-resistant enzymes have been confirmed among the enzymes possessed by thermophiles or hyperthermophiles.
Also with regard to heat-resistant β-glucosidase, it has been identified from several species of thermophiles or hyperthermophiles. Specifically, heat-resistant β-glucosidase has been identified from organisms such as Pyrococcus furiosus, Pyrococcus horikoshii, Thermotoga maritima, Sulfolobus shibatae, and Clostridium thermocellum. 
Cellulase or β-glucosidase derived from filamentous fungi is known to be glycosylated (P. Christian et al., Trichoderma and Gliocladium: Basic Biology, Taxonomy and Genetics, Vol. 1, 121 to 138 (1998)). As a general function of a sugar chain in such a glycosylated protein, effects such as improving protein solubility, improving physical stability, and improving protease resistance are known (H. Ohba et al., Biosci. Biotech. Biochem., 59, 1581 to 1583 (1995)). As a function conferred by the possession of a sugar chain by a saccharification enzyme such as cellulase, it is disclosed that glycosylation of xylanase with N-linked sugar chains results in an increased expression level of xylanase (WO/2005/093072).